a matter of combining a concrete, determinate thing.73 The very notion of combining, in the essence of combination itself, already necessarily entails a pre-viewing of unity.
The next question now is: What is this unity that makes possible combination as such and therefore the understanding itself?  By answering that question we will arrive at the originary a priori of all combining, i.e., of all determining. With that we will arrive at the ultimate a priori of the possibility of determining a manifold as such. And since a manifold as such is determined by the form of time, we will arrive at the most originary possibility—that of determining time as such.74
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§26. The original a priori of all combining—the transcendental unity of apperception
If we are to demonstrate this unity philosophically as the most original, this means we have to demonstrate a “transcendental unity” (Transcendental Deduction, §16, B 132), i.e., one that is the a priori condition of the possibility of the knowledge of nature in general. To comprehend unity as a priori, i.e., to comprehend it in terms of the understanding and its action, means understanding it as a unification. To say unity is prior within the known is correspondingly to say that it is also prior in the subject. That is, unification is the most original action of the understanding—the ur-action of the subject—i.e., as the unifying and combining that makes possible every concrete act of unifying. Combining entails both a manifold as combinable and the pre-viewing of a unity on the basis of which the manifold can be together, i.e., be in a unification.
73.[The complete sentence in the Critique of Pure Reason reads: “We must therefore seek this unity (as qualitative §12) someplace higher, namely in that which itself contains the basis of the unity of different concepts in judgments, and hence [contains the basis] of the possibility of the understanding, even in its logical use.” In §12, Kant had discussed the traditional transcendentalia (unum, verum, bonum, etc.) not as supra-categorial predicates of things but as logical requisites for the cognition of things (B 113–114). He therefore reads unum as the necessary unity not of the thing out-there but of the concept of the thing out-there. He calls this transcendental unity “qualitative unity,” in contrast to the quantitative category “unity” in the Table of Categories (B 95). That transcendental, qualitative unity lies in the “higher place” that is the transcendental unity of apperception.]
74. [Here (Moser, p. 648) Heidegger ends his lecture of Monday, 8 February 1926, to be followed by that of Wednesday, 10 February (Heidegger did not lecture on Tuesday), which opened with a 660-word summary that is omitted in GA 21.]